Support for the XPath true()/false() functions.

lib/oga/xpath/evaluator.rb

@@ -1074,6 +1074,30 @@ module Oga
         return !on_call_boolean(context, expression)
       end
 
+      ##
+      # Processes the `true()` function call.
+      #
+      # This function simply returns the boolean `true`.
+      #
+      # @param [Oga::XPath::NodeSet] context
+      # @return [TrueClass]
+      #
+      def on_call_true(context)
+        return true
+      end
+
+      ##
+      # Processes the `false()` function call.
+      #
+      # This function simply returns the boolean `false`.
+      #
+      # @param [Oga::XPath::NodeSet] context
+      # @return [FalseClass]
+      #
+      def on_call_false(context)
+        return false
+      end
+
       ##
       # Processes an `(int)` node.
       #

spec/oga/xpath/evaluator/calls/false_spec.rb

@@ -0,0 +1,13 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'false() function' do
+    before do
+      @evaluator = described_class.new(parse(''))
+    end
+
+    example 'return false' do
+      @evaluator.evaluate('false()').should == false
+    end
+  end
+end

spec/oga/xpath/evaluator/calls/true_spec.rb

@@ -0,0 +1,13 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'true() function' do
+    before do
+      @evaluator = described_class.new(parse(''))
+    end
+
+    example 'return true' do
+      @evaluator.evaluate('true()').should == true
+    end
+  end
+end